A full session doesn't mean a lost registration — interested people join a waiting list.
A registration for a full session is saved to the waiting list with its position. When a place frees up, the system suggests moving the next person up, and with one click you reserve the place for them — an invoice is issued and the participant receives an e-mail. For a campaign with no session scheduled yet, interested people can leave their e-mail via “let me know”, and you get in touch when you schedule one.